The invention relates to an automatic thread tension adjusting device of a sewing machine.
Generally, a sewing machine has an upper drive shaft rotatable to vertically reciprocate a swingable needle to penetrate a fabric to be sewn and also to reciprocate a thread take-up lever to supply an upper thread from a supply means to the needle and tighten a stitch to be formed. The sewing machine has a loop taker rotatable in association with the upper drive shaft to catch the upper thread for interlocking the upper thread with a lower thread to form the stitch, the upper thread being extended between an upper thread supply and the needle and through at least the thread take-up lever. To tension the upper threads a pair of spring biased disks frictionally clamp it. This clamping pressure is variably adjustable depending on stitch type, fabric thickness, and the type of thread being used. In same designs a computer is used to process these variables so that the clamping pressure is set at the most optimum level for a particular sewing condition. Such an arrangement is usually very complicated and lacks the flexibility to adapt to the many different sewing conditions.
Therefore, the invention is very simple in structure and very reliable in operation, and further may be produced at a lower cost.